The application relates to a method and an apparatus for the production and the melting of liquid pig iron or of liquid steel intermediate products in a melt-down gasifier.
In methods of this type, iron oxides or pre-reduced iron or mixtures thereof are added as iron-containing batch materials to the melt-down gasifier and there are melted, with the supply of carbon-containing material as solid carbon carriers and oxygen-containing gas, in a solid bed which is formed from the solid carbon carriers, the carbon carriers being gasified and a CO- and H2-containing reduction gas being generated. The oxygen-containing gas is supplied to the solid bed via a multiplicity of oxygen nozzles, called an oxygen nozzle girdle, which are distributed over the circumference of the melt-down gasifier in the region of the melt-down gasifier hearth. The oxygen nozzles penetrate through the metal casing of the melt-down gasifier and are supplied with oxygen-containing gas from outside the melt-down gasifier. The oxygen-containing gas may be oxygen or an oxygen-containing gas mixture; the terms “oxygen-containing gas” and “oxygen” are used synonymously below.
The capacity of a melt-down gasifier for producing liquid pig iron or liquid steel intermediate products or its melting capacity increases with its volume. An enlargement of the diameter, that is to say a rising cross-sectional area of the melt-down gasifier, causes the volume to rise for the given height. When the capacity of melt-down gasifiers rises due to an enlargement of the cross-sectional area, the active region of the oxygen nozzle girdle decreases in relation to the cross-sectional area of the melt-down gasifier, since the circumference of the melt-down gasifier hearth grows only linearly with the diameter of the melt-down gasifier hearth, but the cross-sectional area increases with the square of the diameter of the melt-down gasifier hearth. Since, for reasons of the strength of the metal casing of the melt-down gasifier, the spacing of the oxygen nozzles following one another in the oxygen nozzle girdle cannot be made as small as desired, the number of installable oxygen nozzles and also the circumference will increase only linearly with the diameter of the melt-down gasifier hearth, whereas the melting capacity rises at least with the square of the diameter of the melt-down gasifier hearth.
The result of this is that the oxygen nozzles used have to conduct an ever larger quantity of oxygen-containing gas into the melt-down gasifier.
Since the depth of penetration of the oxygen jet into the coke or char bed of the solid bed, which is known as the raceway, in the melt-down gasifier does not become substantially greater with an increasing gas quantity, the disadvantage of a very high local gas quantity arises. Owing to the expansion of the gas jet due to the highly exothermal gasification reactionC+½O2=>COΔH=−110 kJ/molwhich proceeds at temperatures of above 2500° C., the hot gas streams give rise in the raceway and in wide regions above the raceway to a state of fluidized bed formation or fluidization.
In this fluid-dynamic flow regime, solid particles are brought to intensive motion, so that they behave in a similar way to a liquid. For this reason, the countercurrent which is customary in shaft furnaces and is advantageous for energy exchange and mass transfer becomes a cross countercurrent which is unfavorable for the reduction and melting processes taking place in the melt-down gasifier. A further disadvantage is that a pronounced solid bed, which is necessary for the ideal gas-solid countercurrent, no longer occurs in these regions. As a result, material, such as iron ore and sponge iron having different properties, such as degree of reduction and temperature, is intermixed with slags, aggregates and degassed coal (char) which are likewise in different states. A regulated energy exchange and mass transfer is therefore possible only very incompletely.
EP0114040 describes a method in which a fluidization of the material located in front of the oxygen nozzles can be avoided by the arrangement of two nozzle levels. In this case, the lower oxygen nozzle level is supplied with a smaller quantity of oxygen-containing gas, so as to form a solid bed layer which makes it possible to have the process engineering effect of countercurrent management which, as described above, is advantageous for energy exchange and mass transfer. However, by means of this method, only a limited quantity of oxygen-containing gas can be introduced. The oxygen introduced via the upper oxygen nozzle girdle generates a fluidized bed.
A plant according to Austrian patent specification AT382390B possesses only a single oxygen nozzle level issuing into a solid bed consisting of coarse-grained batch material. This method, however, is successful only in the case of hearth diameters up to about 7 m, since, with larger diameters, the initially explained fluidization effect occurs, since the quantity of oxygen-containing gas to be introduced is too large to make it possible to have a stable solid bed. A further limiting criterion is that, when untreated coal is used, this decomposes during pyrolysis into smaller grain sizes which likewise facilitate fluidization.